A multiple-input-multiple-output (MIMO) wireless network is a communication system that generally includes an access point (AP) having multiple antennas communicatively coupled to multiple receiving stations. To support downlink multi-user MIMO network, the access point (AP) may need to schedule multiple packets destined for the multiple stations for simultaneous transmission. Existing IEEE 802.11-207 standards do not support downlink multi-user MIMO. As such, current access points choose the first packet or the first aggregated packet in the access point queue for transmission.
Because an access point has a pre-defined limited number of antennas, it transmits simultaneously to a pre-defined limited number of stations. The pre-defined number is denoted as N. Because the access point cannot simply empty out its queue by transmitting to all stations, a simple scheduling scheme is for the access point (AP) to choose the first N stations' packets to transmit.
The Federal Communication Commission currently limits effective isotropic radiated power (EIRP), when the transmit power is shared among multiple devices, the received power at each individual device is decreased. As an example, if there are two simultaneous transmissions in one downlink multi-user MIMO, the corresponding signal-to-noise ratio (SNR) is reduced by 3 dB. If there are six simultaneous transmissions, the received SNR is reduced by approximately 8 dB. Because the data rate has to be reduced for each of the simultaneous transmissions, the performance gain diminishes if there is a big duration difference among transmissions destined for different stations. As an example, if a transmission toward one station takes significantly longer than those of other stations and the longest transmission has be to sent at a lower data rate, it will have a negative impact on the performance gain of using multiple antennas versus a single antenna system.
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